Emma Stone and Natalie Portman are among hundreds of leading Hollywood women backing a new campaign to tackle sexual harassment.

Time's Up aims to address "the systemic inequality and injustice in the workplace that have kept underrepresented groups from reaching their full potential".

"From movie sets to farm fields to boardrooms alike, we envision nationwide leadership that reflects the world in which we live," says its website.

"Time's Up is a unified call for change from women in entertainment for women everywhere."

It follows the #MeToo movement - which encouraged victims to speak out in the wake of the Weinstein scandal.

Three-hundred prominent women in the entertainment industry have backed Time's Up, including Kerry Washington, Meryl Streep and Shonda Rhimes, the executive producer behind popular television series Grey's Anatomy.

To mark its launch on New Year's Day, the group took out a full page advert in The New York Times and Spanish-language newspaper LA Opinion to publish a letter.

"The struggle for women to break in, to rise up the ranks and to simply be heard and acknowledged in male-dominated workplaces must end; time's up on this impenetrable monopoly," says the letter.

"We particularly want to lift up the voices, power and strength of women working in low-wage industries where the lack of financial stability makes them vulnerable to high rates of gender-based violence and exploitation."

Time Up's open letter is a response to 700,000 farm workers who penned their own letter of solidarity in November.

"We wish that we could say we're shocked to learn that this is such a pervasive problem in your industry. Sadly, we're not surprised because it's a reality we know far too well," it said.

Time's Up will advocate for legislation to tackle workplace harassment across the US and is also backing a movement for women to wear black at this Sunday's Golden Globes, in solidarity with those who have been harassed.